This invention relates to a method and apparatus for the metered supply of a feedstock such as a powder to a powder process operating at high pressures, such as for example a kinetic spray process or gas dynamic method for applying a coating as is described in U.S. Pat. No. 5,302,414.
Powder feeders are employed in many different fields of technology, including plasma spray, petrochemical, kinetic spray, and others. Traditional mechanical delivery powder feeders include mechanical movement for the mechanical delivery of a powder or other feedstock such as a liquid slurry. As part of the mechanical action the feedstock is moved between moving surfaces such as rotating perforated disks, gear teeth, screws or vibrating canisters. For high-pressure mechanical feeders a feedstock is supplied to a stream of high-pressure gas.
In order to operate a kinetic spray process or gas dynamic method of applying a coating, it is required that a feedstock material which is in the form of a powder such as a pure metal, metal alloy or plastic (polymer) or ceramic (typically metal-oxides or metal-carbides) or any combination of the above, be supplied to the process as a stream of powder particles entrained in a carrier gas stream and where the pressure of the carrier gas stream is elevated usually significantly above ambient conditions. Typically the pressure of the carrier gas is between 300-500 psig (20 Bar to 35 Bar).
Known methods and apparatus for supplying a feed of powder particles, entrained in a stream of carrier gas generally are comprised of a canister which contains a supply of the powder and a mechanical mechanism for metering a continuous measured amount of the powder and entraining the powder particles into a carrier gas stream. This mechanical means can include an auger type screw, which is rotated by a motor drive connected to a dynamically moving shaft. Bearings are typically located on one or both sides of the shaft and these bearings must be kept free from powder particles. The motor and bearings make up the drive system for the mechanical powder feeder. The auger screw is located at the end of the shaft. This auger screw is positioned at the bottom of the canister and receives powder from the canister and is metered by the rate of rotation of the screw as well as the size of the screw. The powder from the rotating screw is delivered into a carburetor chamber where it is entrained in a stream of carrier gas. The canister and carburetor chamber are pressurized at usually about 10-20 psi higher than the pressure of the carrier gas. This pressure differential allows the powder to be fed into the main gas stream. A critical dynamic seal is positioned on the shaft between the pressure chamber created by the canister and carburetor and the drive system. This critical dynamic seal prevents the pressure from dropping in the pressure chamber and also prevents the powder material from getting into the bearings surrounding the shaft of the drive system.
Another mechanical means of delivering powder consists of a flat disc containing a series of holes located near the outer perimeter of the disc. This disc is located on an incline within a canister containing the powder. The disc is caused to rotate by a shaft and bearing system attached to a drive motor. As the disc rotates, powder fills the holes in the disc, which carries the powder to the upper portion of the canister where it exits the canister into a carburetor chamber where it is entrained into a carrier gas stream. The action in the carburetor chamber acts to distribute and entrain the powder particles uniformly in the carrier gas stream in both cases. The canister, disc and carburetor are pressurized to create a pressure chamber that prevents the powder from flowing back into the lower part of the canister. A critical dynamic seal is located typically on the shaft of the perforated rotating disc to prevent powder from leaving the mechanical moving portion or disc and getting into the drive system. The seal also maintains the pressure in the pressure chamber.
Another mechanical means consists of powder being fed from a pressurized canister into a continuous annular grove on a rotating metering plate. The rotating metering plate rotates within a chamber. The rotating metering plate and the plate chamber are also maintained at the same pressure as the canister. The rotating metering plate is attached to a shaft and a drive motor drives the shaft. A doctor member ensures that the powder is correctly filled into the groove. The powder is then sucked out of the groove after the plate has been mechanically rotated typically through a 180-degree angle, by a suction device, which has a projection extending into the groove. The powder thus sucked out of the groove leaves the powder feeder entrained in a carrier gas stream. A critical dynamic seal is positioned typically around the shaft to prevent the pressure from dropping and to prevent powder from falling into the drive system from the rotating metering plate and the plate chamber.
U.S. Pat. No. 5,738,249 describes a similar mechanical powder feeder that uses a rotating rotor having quantity measuring recessed parts for collecting powder dropped from a canister. The canister is pressurized and the glands or critical dynamic seals along the shaft of the rotor prevent the powder from falling into the drive system and also maintain the proper pressure in the pressure chamber created in the canister and rotor portion of the device.
The use of any of the above-mentioned mechanical means of powder feeding has the advantage that the means of precisely metering the powder feed rate is readily accomplishable. However, in order to employ any of these mechanical means of powder feeding in a high pressure system it is necessary to use rotating (dynamic) types of pressure and dust seals in order to maintain elevated gas pressures within the mechanical feeder portion and to keep powder from entering into bearing and drive areas. Typically these critical seals are located to block any of the feedstock from entering the drive components of the mechanical delivery powder systems.
FIG. 1 shows a prior art powder feeder system of U.S. Pat. No. 4,227,835 to Nussbaum. In this system a powder canister 15 contains powder. The powder is delivered to a rotating metering plate 1 which rotates within the plate chamber 24. A gas is fed into the powder canister 15 via a feed line 16. This gas pressurizes the powder canister 15 to prevent any dust or powder from blowing up into the canister 15. Critical dynamic seal 21 is positioned to permit the rotation of the metering plate 1 but prevents the powder from entering into the drive components such as the bearings 22 and the driving motor 23. This critical dynamic seal 21 has to function in a rather harsh environment being subjected to abrasive metallic and ceramic dust while also experiencing a pressure differential. This condition, when operating these types of powder feeders at high internal pressures of 20 Bar to 35 Bar tends to leads to early failure of the seals, allowing gas leaks to occur and powder particles to build-up around mechanically moving part of the feeder as well as getting into bearings, thus causing improper function of the powder feeder and resulting in significant downtime and high costs for maintenance. The known practice of employing any of the above mentioned mechanical metering powder feeders to high pressure processes such as kinetic spray is that the canisters are designed and constructed to withstand the 35 Bar pressure and all of the mechanical drive dynamic and static seals are designed and constructed with very special seals which are required to operate at up to 35 Bar pressure.
One aspect of the invention is to provide a method and apparatus for providing a metered supply of a feedstock at elevated pressures, for example between 20 Bar and 35 Bar, which permits reliable and accurately adjustable metering of the feedstock such as powders of different granulation, and continuous uniform feeding of a metered powder to a high pressure powder processing unit. Such a method and apparatus should not require any critical dynamic seal which must experience the pressure differential between the high pressure operation of the mechanical feeding mechanism and the lower ambient pressure surrounding the entire powder feeder thereby resulting in a highly reliable, relatively maintenance free operation.
According to one embodiment of the invention, there is provided an apparatus for the metered supply of powder to a powder-processing unit of the mechanically metered type, in order to provide uniform, precisely controlled powder feed rates. This powder feeder can for example be of any of the mechanically driven types as previously described herein. The mechanically driven powder feeder is enclosed within a pressure vessel with only a group of electrical connections being fed through the pressure vessel wall and requiring only a simple standard high pressure rated electrical feed-thru connector. A tube carrying the high-pressure carrier gas into the pressure vessel and a tube carrying the high pressure carrier gas with the powder entrained out of the pressure vessel and leading to the high pressure powder processing unit are also provided to and from the pressure vessel. In all cases, no critical dynamic seals are required but only simple static type seals are employed to seal off the electrical connections and powder feed hose connection leading from the pressure vessel. No critical dynamic seals are required on the motorized mechanical drives for the powder feeder since they are fully contained within the pressure vessel and are not subject the high differential pressures which they would otherwise be subject to in a more conventional adoption of the mechanical powder feeder for operation at high pressure. In addition there are no special requirements regarding the powder canister since it also is not subject to high-pressure differentials. In a preferred embodiment, the complete powder feeder including all mechanical mechanisms and the powder feed canister are all subjected to an equal pressure equal to that which is maintained within the pressure vessel.
Accordingly, it is an object of the invention to provide a powder feeder, for a system operating above ambient pressure, which has dynamic seals on the dynamically moving portions of the mechanical drives which do not experience a pressure differential.
It is a further object of the invention to provide a high-pressure powder feeder having the dynamic seal associated with preventing powder from leaving the mechanical feeding portion and getting into the drive system from having to operate under a pressure differential.
It is yet another object of the invention to provide a high-pressure powder feeder having the dynamic seal associated with the stirrer from having to operate under a pressure differential.
It is yet a further object of the invention to provide a pressure vessel having a substantially uniform pressure therein. The powder feeder mechanism and drive motors with all dynamic seals associated therewith are within the pressure vessel.
It is yet even a further object of the invention to provide a continuous feed system which isolates the seals associated with the mechanical feeder portion from operating under a pressure differential.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.